In one common approach for opening and closing a valve that controls an opening that communicates with a combustion cylinder of an internal combustion engine, a rocker arm coupled to the valve is pivoted to control the opening and closing of the valve. A valve actuating cam is mounted to a rotating valve cam supporting shaft to engage the rocker arm as the cam shaft is rotated to control the opening and closing of the valve.
An exemplary known valve actuating cam 10 is shown in FIG. 1. Cam 10 has a cross-sectional shape as shown in FIG. 1 that is bounded by a fixed sized and shaped perimeter 12. The illustrated cam 10 has a vertical center line 14 and horizontal center line 16 that intersect at a point 18. The cam includes a base circuit portion consisting of an arc of a circle having a radius 22 from center 18 and as also indicated as R/BC in FIG. 1. The base circuit includes a first base circuit portion 24, from point 1 to point 2 along the cam periphery. Point 1 is located at the intersection of vertical center line 14 and the periphery 12 of the cam at the lower most point shown in FIG. 1. Point 2 is located at a location that is clockwise from point 1 on the cam periphery. The base circuit has a constant radius between these two points 1, 2. The periphery 12 of the cam includes a ramp portion 28 from point 2, in the clockwise direction along the periphery, to a point 3. A radius line 34 indicates the radius R/R of the ramp portion at one location along the arc of the ramp portion between points 2 and 3. The radius R/R is from point R to the periphery. At point 2, the cam shifts the rocker arm to begin to open the valve with a steady increasing acceleration. The cam periphery also includes a flank portion 40 from point 3 to a point 4 in a clockwise direction along the periphery 12 of the cam. At point 3 there is a further increase of the acceleration of the cam follower and therefore of the valve opening. At point 4, the flank portion ends and a first cam top portion 42 begins. The cam top portion extends along the periphery of the cam in a clockwise direction from point 4 to point 5. The illustrated cam top has a constant radius 46 (also designated R/T) from a center point 48 to the periphery 12 of the cam 10. At point 4, where the cam top begins, the acceleration of the valve opening starts to decrease. At point 5, the top of the cam, the acceleration is zero and the valve lift has reached its maximum. Thus, the valve is opened as the contact between the cam and cam following rocker arm moves from point 1 to point 5 due to the rotation of the cam. As the cam moves from point 5 to point 1 the valve is closed. Although the left and right side peripheries of the cam shown in FIG. 1 are mirror images of one another, these cam sections do not need to be symmetrical, which means that different accelerations can be achieved as the valve is opened in comparison to the accelerations as the valve is closed. Moving in a clockwise direction in FIG. 1 from point 5, a second top portion 42′ is provided from point 5 to a point 4′. Also, a second flank portion 40′ is shown between points 4′ and a point 3′. In addition, a second ramp portion 28′ is shown between a point 3′ and a point 2′. Finally, a second base circuit portion 24′ is shown between the point 2′ and the point 1. The ′ (prime) designations have been used to indicate the correspondence between the respective portions of the periphery of the illustrated cam at the right side and left side of the cam.
To avoid acceleration spikes, the ramp portion 28 can start at point 2 along a line that is tangential to the base circuit radius R/BC. In addition, the flank portion 40 can start at point 3 along a line that is tangential to the ramp circuit radius R/R. Also, the top portion 44 can start at point 4 along the line that is tangential to the flank radius R/F at this location.
It is common for three different fixed shapes of a flank portion (points 3 to 4 and 4′ to 3′) to be used for fixed perimeter cams depending upon the cam follower design, the chosen acceleration limit and valve lift achieved by the fixed perimeter cam. In FIG. 1, these three shapes are shown. The most common shape is the flank R/F being of a convex shape as indicated by the radius “R/F convex” and designated by the number 52. If the radius R/F is in effect infinite (see radii 54, 57), the flank is flat as indicated by the dashed line 56 from locations 3″ to 4″. In some cases the flank is designed by selecting a radius R/F to achieve a concave flank, such as indicated by the radius R/F′, and designated by the number 60, at a location of the flank 40′ from location 4′ to location 3′ at the right side of the cam shown in FIG. 1.
Engines that use cams of a fixed peripheral shape suffer from reduced efficiency (specifically in the case of gasoline engines) and increased emissions (e.g., CO2 and NOx emissions). That is, the cam shape is typically optimized for a specified engine speed and load, which leads to inefficiencies as the engine is operated under conditions that are different from the optimized conditions for which the cam shape was designed. For gasoline engines, the load is governed by the combustion air/fuel mixture allowed to enter the cylinder (sucked in or being pressed in with turbocharged engines) during the intake stroke. Consequently, for gasoline engines with cams of a fixed peripheral shape, the engine air inlets are throttled, for example, by a butterfly valve, which closes at idle. In this case, the energy required to introduce the combustion air into a combustion chamber of a cylinder and to exhaust burned gases from the chamber is wasted and reduces the fuel efficiency of the gasoline engine. Consequently, it would be desirable to eliminate such an inlet throttle. To address such issues, attempts have been made to develop gasoline and diesel engines with variable valve actuation. However, variable valve actuated combustion engines known to the inventor are of high complexity, require large space, and are heavier and exhibit significantly higher drag than the non-variable valve actuated engines using a cam of a fixed perimeter such as shown in FIG. 1. The higher drag reduces the achievable efficiency gains.
Therefore, a need exists for an internal combustion engine with improved variable valve actuation as well as for improved components of such a system and related methods.